25. GEOCHEMISTRY OF FLUIDS AND
FLOW REGIME IN THE DÉCOLLEMENT ZONE AT THE NORTHERN
BARBADOS RIDGE 1

M. Kastner,2 Y. Zheng,3
T. Laier,4 W. Jenkins,5 and T. Ito 6

ABSTRACT

The recent three-dimensional seismic reflection
experiment at the décollement zone at the northern
Barbados Ridge subduction zone shows variations in the
amplitude and polarity of the décollement reflection;
this suggests variations in fault-zone velocity and
density, hence in porosity and pore-fluid pressure that
drives the fluid and influences fluid/rock reactions. The
two sites drilled across the décollement zone on Ocean
Drilling Program Leg 156, Sites 948 and 949,
representative of positive and negative polarity
reflections, are located 4 and 2 km, respectively, west
of the deformation front.

Geochemical and temperature depth profiles indicate a
mainly lateral, focused along faults, flow (eastward) of
a warm fluid significantly fresher than seawater at both
the positive and negative polarity sites. The fluid with
the lower than seawater chloride concentration observed
at the top of the décollement zone at Site 948 (~18%
dilution) and enriched in I, Ca, Sr, Ba, Mn, Mo, Zn, and
Co, is advecting from a source region that is situated
deeper than the drilled depths and more arcward. A slight
increase in methane concentration relative to background
level indicates a source region rich in organic matter.
The fluid originates from transformation reactions,
including dehydration, of terrigenous clay minerals as
indicated by the radiogenic strontium and helium-isotope
ratios of the source fluid. In situ volcanic ash
alteration reactions considerably overprint the
geochemical signatures of the end-member low-Cl fluid.
The chemistry and isotopic composition of the carbonate
vein minerals indicate that they have precipitated from a
geochemically distinct fluid from the in situ pore
fluids. Injection of fluid from a deeper and hotter
source caused hydrofracturing and mineralization. The
multiple generations of vein carbonates imply episodic
fluid-flow events, and the strong in situ diagenetic
overprinting of the pore fluids suggests that a major
fluid-flow episode has not occurred recently. The methane
peak, however, signifies active fluid advection.

The pore fluids in the accreted and underthrust
sediments are geochemically distinct, indicating little
vertical advection.